Beating heart surgical methods have the potential to remove the
need for the heart-lung machine and its attendant side effects, but
must contend with the motion of the heart. Recent research in
robotically-assisted surgery has produced a handheld, actuated in-
strument that can track and compensate for heart motion; however,
the reaction forces caused by the actuation mechanism make it dif-
ﬁcult for the surgeon to feel the heart during the operation, which
can lead to unsafe tissue manipulation. This paper investigates an
instrument design that negates reaction forces to the user by moving
a counterweight out of phase with the moving mass of the actuator.
The resulting instrument retains the tracking and motion compensa-
tion abilities of the current instrument, but reduces reaction forces
felt by the user by over 80%. Subjects used the new instrument
in an in vitro beating heart surgical contact task and performance
was compared to the previously existing instrument. The new in-
strument provided a 28% increase in user force sensitivity and im-
proved user reaction times by 51%, indicating that the new instru-
ment greatly enhances force perception in beating heart tasks.